Through normal operations, many electronic components generate electromagnetic emissions that may interfere with other electronic components. Small, passive components such as capacitors, inductors, and ferrites can be used to filter out high-frequency electromagnetic emissions. However, there are situations in which very low-frequency electromagnetic emissions must be eliminated or minimized. For example, radio circuitry in some aircraft platforms are required to be analyzed for electromagnetic emissions all the way down to the audio range of frequencies. This can be problematic when dealing with bursty transmissions and with amplitude modulation (AM) strategies because of the great variation in demand for power. FIG. 1 shows a square radio signal pulse. The power requirement to transmit such a pulse instantaneously increases and decreases at times t1 and t2, respectively, which results in unacceptable electromagnetic emissions in the power supply. If an attempt is made to smooth the pulse beginning at time t1 to reduce the very low frequency electromagnetic emissions of the power supply (as shown in FIG. 2), it is possible that the radio pulse will be unrecognized by a distant receiver awaiting a predefined waveform.
FIG. 3 depicts a known type of very low frequency filter 2 connected to an output of a power supply unit 4. Very low frequency filter 2 relies upon massive energy storage, using very large capacitors 6a and 6b, inductors 8, and other components, to form a very low frequency filter that filters out the bursty power demands of AM transmission. The weight of such inductor- and capacitor-based filters can exceed the weight of the circuitry to be filtered by a factor of two. For airborne solutions or in other applications where weight is at a premium, the increased weight is simply untenable.
It is therefore an object of the invention to filter very low frequency electromagnetic emissions from a power supply.
It is another object of the invention to provide a lightweight, very low frequency filter for a power supply.
A feature of the invention is increasing and decreasing power drawn from a radio power supply input when not required for transmission, while at the same time diverting power in excess of what is required to an actively controlled filter.
An advantage of the invention is that sudden or instantaneous changes in power demand can be smoothed out, thereby substantially reducing or eliminating very low frequency electromagnetic emissions.